Refractories made from a mixture of dead burned magnesite and chrome ore hold an important place in industry. These refractories are generally divided into those which have a predominance of chrome ore and those having a predominance of magnesite. This invention is particularly concerned with those having a predominance of magnesite. These refractories are referred to in the art as magnesite-chrome refractories and will thus be designated in the remainder of this specification.
There are various commercial versions of magnesite-chrome refractories. One type is chemically bonded without any burning or firing treatment. Others are burned. The burned refractories are divided into two groups, which generally are defined as (1) silicate-bonded and (2) direct-bonded. The silicate-bonded refractories are characterized by silicate (forsterite, monticellite, or others) filming about or between the chrome ore and magnesite grains, which filming in a sense glues them together. In the direct-bonded type of refractory, the silicate filming has been minimized or substantially eliminated, so that there is a large degree of direct attachment between adjacent chrome ore and magnesite grains. This invention relates to refractories which are primarily silicate-bonded. However, some degree of direct particle to particle attachments is present.
Both the magnesite-chrome and chrome-magnesite refractories, have their relative advantages and disadvantages. Magnesite-chrome refractories generally are considered more refractory; that is, they will sustain greater compressive loads at elevated temperatures. Magnesite-chrome refractories also have greater volume stability under cyclic temperature or atmospheric conditions, since chrome ores contain oxide which readily release oxygen (are reduced) upon heating and pick up oxygen (are oxidized) upon cooling or upon changing the atmosphere. Chrome magnesite refractories are less expensive because the raw material, chrome ore, is less expensive than high purity magnesite.
Current direct-bonded magnesite-chrome brick have good hot strength at elevated temperatures; however, they expand in burning, which prevents the attainment of low porosity. Magnesite-chrome brick with a high lime to silica ratio shrink in burning and have poor hot strength at elevated temperatures.
Accordingly, it is among the objects of the present invention to provide a dense magnesite-chrome brick with a high hot modulus of rupture at 2700.degree. F, little or no subsidence in the load test after 90 min. at 3100.degree. F under 25 psi, high resistance to slag erosion, low modulus of elasticity, and high spalling resistance.
In accordance with the present invention and in attainment of the foregoing object, there is provided a burned basic refractory shape made from a refractory size graded brick-making batch. The batch comprises dead burned magnesite and chrome ore. The shape has a lime to silica ratio between about 1.7 and 2.1 to 1 and a chromic oxide to aluminum oxide plus iron oxide ratio in excess of 1.5 to 1.
In a preferred embodiment, the batch comprises from about 75 to 90%, by weight, dead burned magnesite, and from about 10 to 25%, by weight, chrome ore. The shape has a chromic oxide to aluminum oxide plus iron oxide ratio in excess of about 1.8 to 1. To insure good high temperature strength, the shape should have a boron oxide content of less than about 0.02%. To insure good resistance to spalling, the shape should have an R.sub.2 O.sub.3 content in excess of about 19%. The term R.sub.2 O.sub.3 is used to collectively include Cr.sub.2 O.sub.3, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3.
The shapes may contain a mixture of chrome ores and magnesites, if desired. They may also contain an addition of chromic oxide. It is preferred that the chrome ores be sized so that they are substantially all -10 mesh, and that the chromic oxide employed be a very fine pigment grade.
In accordance with the stated objects, the properties sought to be obtained are set forth below:
a. High temperature modulus of rupture (MOR) at 2700.degree. F -- greater than 1000 psi. PA1 b. Good resistance to subsidence -- in the load test less than 0.5% subsidence after 90 min. at 3100.degree. F under 25 psi load. PA1 c. Good resistance to slag erosion -- less than 4% erosion in AOD slag test. Less than 1% erosion in electric furnace slag test. PA1 d. Low modulus of elasticity -- less than 5.times.10.sup.6 psi and preferably less than 3.times.10.sup.6 psi. PA1 e. High resistance to spalling -- in the prism spalling test, no breakage after 15 cycles and preferably no breakage after 30.
Some brick will not meet all of these desired properties, but will still be excellent brick and within scope of this invention.
In the following examples, all mesh sizes are according to the Tyler series. All parts and percentages are by weight. The chemical analyses of all materials are on the basis of an oxide analysis, in conformity with common practice in reporting the chemical content of refractory materials. The various chemical constituents are reported as though they were present as the simple oxides.